1. Technical Field
The present disclosure relates to a CO2 separator and a direct methanol fuel cell including the same.
2. Description of the Related Art
A fuel cell is a galvanic cell that converts chemical reaction energy of fuel and oxidant that are continuously fed thereto into electrical energy. In general, a fuel cell includes two electrodes that are separated by a membrane or an electrolyte. An anode is contacted by a flow of fuel, for example, hydrogen, methane, or methanol, and the fuel is oxidized at the anode. A cathode is contacted by a flow of an oxidant, for example, oxygen, hydrogen peroxide, or potassium thiocyanate, which is reduced at the cathode. Materials used for each component are typically selected depending on the type of the fuel cell.
A direct methanol fuel cell (DMFC) is a low-temperature fuel cell that typically operates within a low-temperature range of about 60° C. to 120° C. This kind of fuel cell typically uses a polymer membrane as an electrolyte. Methanol (CH3OH) that is not reformed in advance is directly supplied to the anode with water, and is oxidized at the anode. Carbon dioxide (CO2) is formed at the anode as a waste gas. Atmospheric oxygen supplied to the cathode as the oxidant reacts with H+ ions and electrons to form water. The DMFC has several advantages, including a liquid fuel that can be stored, for example, in a plastic cartridge, which is easily stored, and which is a very inexpensive energy source. Moreover, an extensive infrastructure for methanol already exists in many fields, for example, as an anti-freeze additive in windshield washer fluids for vehicles. This type of fuel cell can provide power ranging from a few mW to hundreds of kW depending on the design of the fuel cell. In more detail, the DMFCs are suitable for portable use as substitutes or supplements for typical power sources in electronic devices. Typically, the DMFCs are used in communications and power supply of laptop computers.
Because oxidation of methanol proceeds gradually on the catalyst of the anode, various reaction pathways having various intermediate products are significant. In order to maintain a high efficiency of the fuel cell, reaction products should be rapidly removed from peripheral regions of the electrode. A liquid/gas mixture of CO2, water, vapor, and unreacted methanol is formed as a result of a chemical reaction at a predetermined temperature, the mixture on which is the following discussion is based.
Therefore, a CO2 separator mainly keeps water and removes CO2 from the equilibrium mixture. In general, the CO2 separator is installed as an additional apparatus that is connected to the fuel cell through a feed line that is common for the liquid/gas mixture. A spatial distance causes a temperature gradient, and the water condenses from the liquid/gas mixture that is cooled down slowly. The typical CO2 separator separates the liquid and gas phases of the liquid/gas mixture and discharges the gaseous component, for example, into the outside environment.
The typical CO2 separator has a porous membrane for separating the liquid/gas mixture. An inside of the porous membrane faces the liquid/gas mixture, and an outside of the porous membrane contacts with the outside environment. Moreover, the porous membrane is generally coated with a hydrophobic material, or can be formed of the hydrophobic material. Diffusion channels extend from the inside of the porous membrane to the outside of the porous membrane, and the channel has a size through which the liquid cannot permeate, but CO2 can diffused to the outside thereof.
One of the disadvantages of the typical CO2 separator is that a high level of vapor exists in the gas phase of the liquid/gas mixture because the temperature of the liquid/gas mixture is in a range of from about 60° C. to 80° C. when the mixture enters the CO2 separator. However, the gaseous component of the liquid/gas mixture is further cooled while passing through the porous membrane, and condenses water therein. The condensed water blocks the diffusion channels/pores, thereby reducing the passage of CO2 therethrough or completely preventing the passage of CO2 therethrough in the worst case.